The term actuator refers to a member of a class of mechanisms whose primary function is to provide a controllable force for moving an actuated element to a desired position. An actuator system typically includes an actuator, an actuated element, connecting linkage and a power source. Some actuator systems must also perform a force limiting function to prevent damage should the system become jammed. This is sometimes done through torque limiting devices on the actuator input shaft. An aircraft actuation system is an example of a system which may use a torque limiter as a force limiter for an actuator.
Numerous aircraft actuation systems utilize gear rotary actuators that contain torque limiters. On these systems, the torque limiter protects actuator reduction gearing and the downstream aircraft structure by limiting the actuator output torque to a predefined value in the event of an overload or a jam. These traditional torque limiters measure the torque that passes through them. This torque must include actuator drag torque due to cold temperature operation and sufficient torque to ensure operation with the minimum actuator efficiency under the maximum loaded condition. A maximum torque limiter setting for the actuator is determined based on these considerations. This maximum torque limiter setting is generally the limit load which can be imposed on the actuator. Actuator size, weight and envelope are determined from this maximum torque limiter setting.